Non-injecting heroin use is widespread in New York City and in many other cities. Non-injecting heroin users (NIUs) are at high risk of becoming injecting drug users (IDUs). Increases in the number of new IDUs can lead to many new infections with HIV, hepatitis B (HBV) and hepatitis C (HCV). This study will continue the "HIV Risk and Transitions from Non- Injecting Heroin Use" (NIU) cohort study of approximately 600 NIUs. It will follow the NIU cohort and a cohort of new IDUs, to determine: (1) the incidence and time-trends in transitions to injecting; (2) behavioral and network risks for transitions to injecting and other drug use outcomes; (3) separately, for new IDUs and for NIUs, the incidence and time-trends in HIV, HBV and HCV seroconversions; (4) among NIUs, sexual and other non- injecting risk behaviors and networks predicting HIV, HBV and HCV seroconversion; (5) among new IDUs, drug injecting and sexual risk behaviors and risk networks predicting HIV, HBV and HCV seroconversion; (6) time-trends in the prevalence of drug and sexual risk behaviors and risk networks, and factors predicting these trends; and (7) the impact of changes in the quality and availability of heroin, and in other community factors, on modes of drug administration and other drug and sexual risk behaviors and risk networks. The sample will include: NIUs from the parent study; new IDUs recruited from a supplemental study of network risks for HIV, HBV and HCV infection; and NIUs and new IDUs recruited during the study continuation. Participants will be interviewed and tested for HIV, HBV, and HCV. Ethnography will be used to study the impact of drug markets. Analyses of risk factors for transitions to injecting and seroconversions will explore the "network facilitation" hypothesis by examining the extent to which network characteristics, in interaction with risk behaviors, facilitate transitions to injecting or the cessation of heroin use and other drug use outcomes, and seroconversions. This approach will advance our knowledge of the interaction between network processes and risk behaviors, and can be used to develop interventions to change both risk behaviors and risk networks, and to prevent transitions to injecting and the risk of HIV, HBV and HCV infection.